1. Field of the Invention
The present invention relates to a switching mode power supply. More particularly, the present invention relates to a transformer-based flyback converter employing secondary pulse width modulation control and having a primary side start-up circuit powered by voltage supplied from the secondary side.
2. Introduction to the Invention
The present invention relates to electronic switching power supplies in high input voltage, low power applications, such as off-line battery charging circuits that require self-contained bias power derived from the input-side AC mains. For safety reasons it is necessary to provide electrical isolation between the input mains and the output power of a switching power converter. In AC mains powered switching power converters, output isolation is conventionally accomplished by providing a transformer between the input side and the output side of the converter. The high voltage switching element and the pulse width modulation (PWM) control circuit are typically implemented on the primary side of the transformer. To regulate the output voltage or output current, or both, one or more feedback loops are provided for coupling control values from the output side to the input side control circuit. Due to the need for isolation, the feedback paths from output side to input side also have to be isolated. Isolation of the control values is frequently achieved by employing optical coupling via an optical isolator assembly, or by induction via a control transformer. The signal transmitted across the isolation barrier is usually an analog signal, and as such, is susceptible to noise and parameter drift due to temperature variation, distortion due to isolation circuit nonlinearities, and bandwidth limits of the isolation circuit or component.
Based upon the foregoing reasons, a secondary-side control circuit may be incorporated into a switching power supply. In using secondary-side control, the PWM control circuit is implemented entirely on the secondary side, while the electronic switch element is on the primary side. Since all output voltage or current sensing is carried out on the secondary side, there is no need to transfer analog control signals across the isolation barrier. Rather, the control circuit generates an on-off pulse-width-modulated control sequence which is coupled to the primary side switch element through a pulse transformer, for example. Because direct connection is made to the AC mains on the primary side, there is no power readily available at the secondary side PWM control circuit at start-up. Thus, special provision must be made to ensure that the power supply will begin switching when power is first applied via the AC mains.
FIG. 1 illustrates an example of a conventional switching power supply 20 having a secondary side control. The supply 20 includes an input side 21 and an output side 22, separated by a switching power transformer 17 having a primary winding 4 and two secondary windings 5 and 6. The primary winding 4 is connected to a high frequency inverter 2, which in turn is connected to an input filter and polarity protection (rectifier) circuit 1 in direct connection with the AC mains. During operation of the supply 20, a switching element within the converter circuit 2 causes an alternating current to flow through the primary winding 4, and currents are induced in secondary windings 5 and 6. An output rectifier and filter circuit 7 is connected to the secondary side 6 and rectifies the induced AC power in order to provide DC power output at desired voltage and current levels.
In order to regulate the output of the circuit 7 to the desired levels a control circuit 15 is provided. In the FIG. 1 example, the control circuit 15 includes a primary side control circuit 12 which generates a startup switch waveform, and a secondary control circuit 14 which generates a PWM control signal regulated by feedback control. A pulse transformer 16 provides primary/secondary side isolation and couples the PWM control signal from the secondary control circuit 14 to the high frequency inverter circuit 2 via a control path 13. A primary side on-off switch 10 bypasses the primary control startup circuit 12, and/or a secondary side on-off switch 11 bypasses the secondary control circuit 14. Switches 10 and/or 11 may be provided to control startup and shutdown operations of the supply 20.
In order to provide initial startup, the primary control startup circuit 12 derives operating power through a resistor R1 from a DC bus between rectifier 1 and inverter 2. The primary control startup circuit 12 puts out square wave switching control signals over a path 3 to the inverter 2 which bypasses the pulse transformer 16 in order to control the high frequency inverter circuit 2 during startup. After startup, a feedback signal from the secondary winding 5 will cause the primary control circuit 12 to stop sending the square wave switching signals when sufficient energy is being transferred to the secondary winding 6 to operate the secondary control circuit 14. From this point on, the secondary control circuit 14 will take over all switching control of inverter 2 via control path 13 and feedback isolation pulse transformer 16. The secondary control circuit 14 performs conventional voltage regulation by comparing output voltage level with a predetermined reference in order to adjust the on-off duty cycle of the switching element of the high frequency inverter 2. Power transformer 17 is typically, although not necessarily, a step-down transformer. A low voltage induced in secondary winding 6 provides power to the output rectifier and filter circuit 7 which in turn provides a smooth, regulated DC voltage at the output.
Since there is no isolation component in a feedback control line 8 from the output to the secondary PWM control circuit 14, the limitations noted above with analog signal isolation are not present. However, startup power for the secondary control circuit 14 is more difficult to acquire, as compared with the conventional primary side control scheme, where the entire control circuit is present on the primary side of the power transformer. One typical approach is to include an electronics circuit to generate a PWM signal with a fixed frequency and duty cycle, or a square wave, in order to cause transfer of start-up power to the secondary control circuit 14. Since this start-up electronics circuit 12 is on the primary side, the components may be subject to high voltage stress from the AC mains, and a high voltage silicon integrated circuit process may be required to implement the start-up circuit 12.
From a reliability standpoint, it is desirable to limit silicon components on the primary side to rectifiers and the switching element in inverter 2. Other concerns and drawbacks include added cost and complexity to provide effective startup circuitry.
A general object of the present invention is to provide an isolated output, switching mode power supply which includes a simplified input side starting circuit and a low voltage output side integrated control circuit which overcomes limitations and drawbacks of prior approaches.
One more general object of the present invention is to provide an isolated output, switching mode power supply which includes a starting circuit employing self-oscillation during an initial startup interval and a low voltage output side integrated control circuit which takes over control of the starting circuit as soon as secondary side power becomes available, in a manner overcoming limitations and drawbacks of prior approaches.
A third general object of the present invention is to provide a switching mode battery charger circuit which starts up and operates reliably over a wide variety of AC mains voltages present throughout the world.
Yet a fourth general object of the present invention is to provide a low voltage integrated circuit for controlling a switching mode power supply from a secondary side of said power supply in a manner overcoming limitations and drawbacks of prior approaches.
In one aspect the present invention provides an isolated-output switching power supply having a transformer with a primary winding and at least one secondary winding. A first rectifier-filter rectifies and smoothes input power drawn from the AC mains. A series network including the primary winding and a source-drain path of a switching field effect transistor enables energy to be switched into a core of the transformer. A starting circuit including a first resistor-capacitor network is connected to apply a declining voltage level derived from the rectified input power directly to a gate of the transistor during initial power-on, so that the transistor conducts and transfers input power through the primary and into the core until a time constant of the resistor-capacitor network causes the transistor to stop conduction. When conduction through the primary winding stops, energy stored in the core is transferred to the secondary winding. A second rectifier and small value smoothing capacitor are connected to the secondary winding to produce an initial operating low voltage. An integrated control circuit chip is electrically configured and connected to receive and use the initial operating low voltage to begin generating and putting out switching pulses to the gate of the transistor through an isolation circuit so that regulated switching of the transistor occurs immediately after the transistor has stopped conduction in accordance with the initial declining voltage level. In this aspect of the invention the transformer most preferably has a second secondary winding and the power supply further includes a third rectifier for producing a second secondary voltage. A current-limiting network comprising a third capacitor, a first inductor, and a fourth smoothing capacitor initially isolates an output load from the second secondary winding during initial startup while thereafter filters and provides the second secondary voltage as regulated DC power to the load. As one more aspect of the present invention, an output level monitor is connected in a network including the second secondary winding and third rectifier, and the integrated control circuit chip is electrically connected to the output level monitor and regulates duty cycle of the switching pulses in relation to monitored output level of the power supply flowing to the load.
In another aspect of the present invention, an isolated-output switching power supply comprises a transformer having a primary winding and a secondary winding. A first rectifier rectifies input power from AC mains. A series network includes the primary winding and a source-drain path of a switching field effect transistor. A resonant circuit network is connected to a gate of the transistor to cause the transistor to self-oscillate (switch) during an initial power-on interval so that the transistor transfers input alternating current through the primary and into a core of the transformer. The energy stored in the core of the transformer thereupon is transferred to the secondary winding. A second rectifier and a small value smoothing capacitor are connected to said secondary winding to produce an initial operating low voltage. An integrated control circuit chip is electrically connected to receive and use said initial operating low voltage to begin generating and putting out switching pulses. An isolation circuit includes a pulse transformer having a secondary forming a part of the resonant circuit network and transfers the switching pulses to the gate of the transistor and causes the transistor to stop self-oscillation following the initial power-on interval.
In a related aspect of the invention, a low voltage switching current control integrated circuit is provided for use within a switching power supply having an input side isolated from an output side by a power transformer. The primary side includes a primary winding of the power transformer, a first rectifier and filter for rectifying and smoothing alternating current from power mains to provide primary direct current, a MOSFET switch having a source and drain current path in series with the primary winding and having a gate circuit, starting circuit means for causing the MOSFET switch to conduct initially and transfer energy into a core of the power transformer during an initial startup interval. The isolated secondary side includes at least a first secondary network having a first secondary winding and a second rectifier and filter for rectifying and smoothing said energy into a low level operating voltage. The low voltage current control integrated circuit generates control pulses for controlling the gate circuit upon receiving the low level operating voltage. The secondary side most preferably further includes a second secondary network having a second secondary winding and a third rectifier, isolator and filter for rectifying, initially isolating during the initial startup interval and then filtering and smoothing energy from the transformer into an output power for application to an external load. In accordance with this aspect of the present invention, the integrated circuit includes:
(a) a low level operating voltage monitoring circuit connected to monitor the level of operating voltage supplied from said first secondary network,
(b) a linear filtering control circuit connected to add capacitance of an external capacitor to the second rectifier and filter as operating voltage level increases during the initial startup interval,
(c) an output power monitoring circuit for monitoring the output power for application to the external load, and
(d) a width-modulated pulse generator circuit for generating recurrent control pulses having widths controlled by monitored output power, the control pulses for application through an isolation circuit, such as a blocking capacitor and pulse transformer, to the gate of the MOSFET switch.
In this aspect of the invention the output power monitor circuit most preferably includes a voltage monitor and a current monitor.
These and other objects, advantages, aspects and features of the present invention will be more fully understood and appreciated by those skilled in the art upon consideration of the following detailed description of preferred embodiments, presented in conjunction with the accompanying drawings.